1. Field of the Invention
The invention relates generally to mine roof testing methods and apparatus and, more particularly, but not by way of limitation, it relates to improved testing apparatus for determining mine roof bolt tension as well as general mine roof stability.
2. Description of the Prior Art
The prior art has seen numerous types of tensometers and similar devices for making some determination as to the secure positioning and rigidity of a metallic object. Prior art patents which are particularly related to testing of a metal bolt or the like have primarily relied upon ultrasonic energy behavior in deriving parameter indicators illustrating rigidity or reliability. U.S. Pat. No. 3,307,393 in the name of Kessler teaches a stress measuring device wherein determination is made by measuring a change in electrical impedance due to an increase in length of the fastener. This method provided a marked degree of improvement over the prior methods of simply measuring the degree of torque as applied to the fastener in seating it to operative position. U.S. Pat. No. 3,306,100 in the name of Wilhelm et al. discloses yet another form of ultrasonic bolt tension tester which utilizes as an indicator, frequency as a function of the length of the fastener, Young's modulus of elasticity and the density of the fastener; however, there is no direct relationship as to the tension of the fastener contributing to the final readout. The method measures the changes in length and then interprets increased length to increased tension of the fastener.
U.S. Pat. No. 3,759,090 in the name of McFaul et al. provides yet another and more recent ultrasonic tensometer wherein the method serves to measure the elongation of a bolt due to tension and thereafter extrapolates the magnitude of tension. Finally, a recently issued U.S. Pat. No. 4,062,229 as issued on Dec. 13, 1977 makes disclosure of a method similar to that set forth and claimed in the present application.